Field of the Invention
The present invention relates to a method for the continuous manufacture of powder-filled tubular welding electrodes and to an apparatus for their production. Such electrodes can, for example, be used for automatic or semiautomatic arc welding of metal pieces with or without shielding gas or under flux.
In the prior art, such a method previously included steps of continuously feeding a metal strip means at a predetermined speed, forming the strip means into an open channel with edges extending longitudinally from the channel and forming a tube by abutting the edges, then discharging welding powder into the tube from a filling station, and then passing said tube through a welding station at which the abutting edges are continuously welded to each other at the predetermined speed by heating the abutting edges at a welding spot or location. The welding powder is passed into the tube through the welding station at some distance from the welding spot or location and thereafter the cross-sectional area of the welded and powder-filled tube is reduced to a desired dimension. Prior to forming, the metal strip means may, for example, have the shape of extruded wire or metal strip. During manufacture, the metal strip means is also represented by the open channel and the tube.
The advantages featured by this type of tubular electrode with a welded tube, i.e. seamless cover, are numerous compared to a tubular covering comprising a strip, which has as yet only been closed by menas of bending and folding, thus resulting in an open joint. The seamless electrode features absolute protection against deleterious hygroscopic absorption. As a result, it may be subjected to cleaning in cleaning baths and to copperplating in, e.g., galvanic baths during manufacture.
Several proposals relative to the production of such electrodes have been known. According to the U.K. Patent Specification No. 1 177 993 - a prewelded tube is wound on to a coil having a vertical axis which is filled with welding powder through one of its feed ends. Then by simultaneously shaking the entire coil, the powder is thereby evenly distributed throughout the tube and fills the entire volume. This manufacturing process, which is not continuous, is quite complicated. It requires prefabrication and precoiling of the tube before it can be filled with welding powder.
According to another idea - U.K. Patent Specification No. 1 485 571 - a strip formed to make an open channel is filled with welding powder before the edges of the channel are welded together. Simultaneously keeping pace with the feed speed of the tube, a strip is inserted into the tube which acts as a backing strip during the welding of the edges of the tube. The backing strip protects the welding powder against overheating by the welding heat. According to still another idea - European Patent Application No. 003 370 - the open channel is filled with a limited amount of powder prior to being closed into a tube. Consequently, only part of the cross-sectional area of the tube will be filled with powder as the tube passes the welding station. The powder passes the weld at the same speed as the tube but at some distance from the weld spot or location, thereby preventing to some degree the deleterious influence of the welding heat on the powder. However, relative to the aforementioned process, there is some risk of powder damage due to overexposure to welding heat. The accumulation of powder at the weld is difficult to control, especially as the magnetic field produced by the welding current greatly influences the somewhat ferromagnetic powder. The magnetic field also causes an uneven powder distribution inside the tube, thus impairing the welding properties of the electrode.